Role of tip shape in light emission from the scanning tunneling microscope

The influence of tip shape on the light produced by scanning tunneling microscopy is analyzed theoretically for the case of nobel metals where collective modes enhance the photon emission. We investigate a hyperbolic tip geometry where the aperture of the tip and its apex curvature can be changed independently. The electromagnetic field in the tip-sample region is calculated with the use of the boundary charge method. The tunneling current is treated in a modified Tersoff-Hamann theory. The aperture of the tip is found to control the overall shape of the emission spectrum, while the radius of curvature of the apex is more important for the intensity. Experimentally observed variations of emission spectra may be understood in terms of different tip shapes. The lateral extent of the tip-induced charge density is strongly dependent on the tip shape, and may reach a near-atomic scale for sufficiently sharp tips. This spatial localization has direct implications for the resolution in photonic maps.